In the past, photographic processes using silver halide have been most widely used because they are excellent in photographic properties such as sensitivity or control of gradation as compared with other photographic processes, such as, an electrophotographic process or a diazo photographic process. In recent years, with respect to image formation processes for photographic materials using silver halide, many techniques capable of obtaining images with ease and rapidly have been developed by changing the conventional wet process using a developing solution into a dry process such as a process using heat.
Heat-developable photographic materials are known in the field of these techniques. Heat-developable photographic materials and processes therefor have been described in U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,457,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, No. 17029, pages 9 to 15 (June 1978).
Many different processes for obtaining color images have been proposed. With respect to processes for forming color images by the reaction of an oxidation product of a developing agent with a couplers, the following processes have been proposed: (1) using a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler as described in U.S. Pat. No. 3,531,286; (2) using a p-aminophenol type reducing agent as described in U.S. Pat. No. 3,761,270; (3) using a sulfonamidophenol type reducing agent as described in Belgium Pat. No. 802,519 and Research Disclosure, pages 31 and 32 (September 1975); and (4) using the combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler as described in U.S. Pat. No. 4,021,240. These processes, however, are disadvantageous in that turbid color images are formed, because a reduced silver image and a color image are simultaneously formed on the exposed area after heat development. In order to eliminate these disadvantages, there have been proposed a process which comprises removing a silver image by liquid processing as well as a process which comprises transferring only the dye to another layer, for example, a sheet having an image receiving layer. However, the latter process is not desirable because it is not easy to transfer only the dye as distinguishable from unreacted substances.
Another process which comprises introducing a nitrogen-containing heterocyclic group into a dye, forming a silver salt and releasing a dye by heat development has been described in Research Disclosure, No. 16966, pages 54 to 58 (May 1978). According to this process, clear images can not be obtained, because it is difficult to control the release of dyes from unexposed areas, and thus it is not a practical process.
Processes for forming a positive color image by a thermal silver dye bleach process, with useful dyes and methods for bleaching have been described, for example, in Research Disclosure, No. 14433, pages 30 to 32 (April 1976), ibid., No. 15227, pages 14 to 15 (December 1976) and U.S. Pat. No. 4,235,957. However, this process requires an additional step and an additional material in order to accelerate the bleaching of dyes, for example, by heating with a superposed activating agent sheet. Furthermore, it is not desirable because the resulting color images are gradually reduced and bleached by coexisting free silver during preservation for a long period of time.
A process for forming a color image utilizing a leuco dye has been described, for example, in U.S. Pat. Nos. 3,985,565 and 4,022,617. However, this process is not desirable because it is difficult to stably incorporate the leuco dye in the photographic material and coloration gradually occurs during preservation.
It is also known in the field of textiles to dye polyester with an azo dye. Although processes for application of the dyeing of polyester to a photographic field have been proposed, sufficient results have not been obtained. For example, a process for using a polyester film as an image receiving layer is described in U.S. Pat. No. 4,235,957. However, according to this process, the diffusion of dyes to a polyester film and the bleaching of dyes occurs simultaneously in order to form a positive image by a silver dye bleach process, and thus it is difficult to form an image having a good S/N value.
A process for transferring a dye formed upon the coupling reaction of a hydrazone developing agent with a coupler into a polyester support is described in the example of British Pat. No. 2,056,103. However, this process does not provide a sufficient image since a dye forming efficiency of the coupling reaction is low and also a transferring property to the support is poor.